1. Field of the Invention
The present invention relates to a device for detecting insulation deterioration in a plurality of motors connected to a motor driving apparatus capable of driving a plurality of motors.
2. Description of the Related Art
In JP-2006-226993A, there is described a device for detecting insulation deterioration in the windings of a motor connected to a motor driving apparatus comprising a power supply unit for producing DC power by rectifying three-phase AC power and smoothing it by a smoothing capacitor and a motor driving amplifier for inverting the DC power into AC power whose frequency is variable and for driving the motor. The device detects insulation deterioration by using an electric charge that remains in the smoothing capacitor for a finite period after the three-phase AC power has been shut off. That is, when detecting insulation deterioration, the three-phase AC power is shut off, and one end of the smoothing capacitor is connected to ground and the other end to the motor windings. Then, the discharge current flowing via the insulation resistance of the motor is compared with a reference value to detect the presence or absence of insulation deterioration.
The degree of motor insulation deterioration is judged with reference, for example, to 10 MΩ, and if the insulation resistance is smaller than this reference value, it is determined that the insulation has deteriorated. In the above device, the insulation deterioration is not judged by directly measuring the insulation resistance, but is judged by comparing the discharge current with the reference value, assuming that the voltage across the smoothing capacitor remains constant. However, the voltage across the smoothing capacitor varies according to the supply voltage, and changes due to the discharge occurring after the three-phase AC power has been shut off. Furthermore, the speed of change also varies according to the size of the load. It is therefore not possible to accurately detect insulation deterioration based on the assumption that the voltage across the smoothing capacitor remains constant, and a false warning of insulation deterioration may be output.
One possible method to address this problem would be to measure the voltage across the smoothing capacitor as well as the discharge current, to calculate the insulation resistance from them, and to compare it with the reference value.
However, in the case of a motor driving apparatus capable of simultaneously driving multiple motors, there arises the problem that such a measuring circuit has to be provided for each motor to be monitored and the configuration of the apparatus thus becomes redundant.